I. Field of the Invention
The present invention relates generally to mobile radio communication systems. More particularly, the present invention relates to code division multiple access (CDMA) mobile radio communication systems that control the transmission rate of data information signals sent on the forward link from a base station to a mobile unit. Even more particularly, the present invention relates to a novel and improved system and method for controlling the transmission rate of data on the forward link by alternatively using variable and fixed rate data transmission modes to transmit data from a selected base station to the mobile station.
II. Description of the Related Art
In mobile radio communication systems, there are significant differences between the requirements for providing voice and data services (i.e., non-voice services such as internet or fax transmissions). Unlike data services, voice services require stringent and fixed delays between speech frames. Typically, the overall one-way delay of speech frames used for transmitting voice information must be less than 100 msec. By contrast, transmission delays that occur during data (i.e., non-voice information) services can vary and larger delays then those that can be tolerated for voice services can be utilized.
Another significant difference between voice and data services is that, in contrast to data services, voice services require a fixed and common grade of service. Typically, for digital systems providing voice services, this requirement is met by using a fixed and equal transmission rate for all users and a maximum tolerable error rate for speech frames. For data services, the grade of service can vary from user to user.
Yet another difference between voice services and data services is that voice services require a reliable communication link which, in the case of a CDMA communication system, is provided using a soft handoff. A soft handoff requires the redundant transmission of the same voice information from two or more base stations to improve reliability. This additional reliability is not required to support data services, because data packets received in error can be transmitted.
As a mobile station moves in a mobile radio communication system, the quality of the forward link (and the capacity of the forward link to transmit data) will vary. Thus, at some moments a given forward link between a base station and a mobile station will be able to support a very high data transmission and, at other moments, the same forward link may only be able to support a much reduced data transmission rate. In order to maximize the throughput of information on the forward link, it would be desirable if the transmission of data on the forward link could be varied so as to increase the data rate during those intervals where the forward link can support a higher transmission rate.
When non-voice traffic is being sent from a base station to a mobile station on a forward link, it may be necessary to send control information from the mobile station to the base station. At times, however, even though the forward link signal may be strong, the reverse link signal may be weak, thereby resulting in a situation where the base station cannot receive control information from the mobile station. In such situations, where the forward link and the reverse link are unbalanced, it may be undesirable to increase the transmit power on the reverse link in order to improve the reception quality of the control information at the base station. For example, in CDMA systems, increasing the transmit power on the reverse link would be undesirable, as such a power increase could adversely affect the reverse link capacity seen by other mobile stations in the system. It would be desirable to have a data transmission system where the forward and reverse links associated with each mobile station were maintained in a balanced state without adversely impacting the reverse link capacity. It would be further desirable if such a system could maximize the throughput of non-voice data on individual forward links when such links are sufficiently strong to support higher data rates.
The present invention is directed to a method and apparatus for controlling a data rate associated with the transmission of information from a base station to a mobile station in a mobile radio communication system. The mobile station alternately receives information from a base station either in a variable rate mode or a fixed rate mode. The transmission rate from a base station in the variable rate mode varies between successive data transmit intervals, and the transmission rate from a base station in the fixed rate mode remains fixed between successive data transmit intervals. In order to maximize throughput on the forward link, the base station preferably uses the variable rate transmission mode to increase the data transmission rate whenever the forward link can support higher rates. The base station monitors control information sent from the mobile station on the reverse link, and the base station uses this control information to indicate to the mobile station when the reverse link is weak and in an unbalanced state with respect to the forward link. When the forward link is operating in the preferred variable rate mode and the mobile station determines that the reverse link is in an unbalanced state, the mobile station first attempts to locate a second base station that can operate in the variable rate mode and support a high data rate to the mobile station. If the mobile station locates such a second base station, then data transmissions continue from the second base station in the variable rate mode; otherwise, data transmissions continue from the first base station or a different base station in the fixed rate mode. The mobile unit then attempts to return to the variable rate mode when there is a forward link that is sufficiently strong to support data transmissions in the variable rate mode.
In a particularly preferred embodiment, data is transmitted from a first base station to the mobile station in the variable rate mode until the first base station is unable to receive variable data rate control information from the mobile station. The variable rate control information includes, for example, the data rate that the base station should use for transmitting data to the mobile station during the next transmit interval. When the first base station is unable to receive the variable data rate control information from the mobile station (thereby indicating that the current forward and reverse links are unbalanced), the mobile station attempts to remain in the variable rate mode by searching for a second base station that will support high data transmissions to the mobile station in the variable rate mode. In order to be selected, the second base station must have a long term average forward link signal to noise ratio that is not significantly less than the long term average signal to noise ratio associated with the forward link from the first base station. If the mobile station is able to locate the second base station that will support high data transmissions to the mobile station in the variable rate mode, then the second base station transmits data to the mobile station in the variable rate mode and transmissions from the first base station to the mobile station in the variable rate mode terminate. If the mobile station is unable to locate the second base station that will support data transmissions to the mobile station in the variable rate mode, data is transmitted to the mobile station from a designated base station (i.e., either the first base station or a different base station) in the fixed rate mode.
The mobile station will switch from the fixed rate mode back to the variable rate mode either (i) when the designated base station used for transmitting data to the mobile station in the fixed rate mode can again reliably receive the variable rate control information and the forward link signal received at the mobile station from the designated base station has a long term average signal to noise ratio that exceeds a threshold, or (ii) when the mobile station receives a signal from a further base station (other than the designated base station used for transmitting data to the mobile station in the fixed rate mode) with a long term average signal to noise ratio that exceeds by a threshold the long term signal to noise ratio needed on the forward link signal to support data transmissions in the fixed rate mode.